Absorbent members are arranged at areas receiving body fluids such as urine and blood of menstruation, in sanitary goods such as disposable diaper and sanitary napkin. The absorbent members have a structure such that pulp or a super absorbent polymer (referred to as "water-retentive material" hereinafter) is interposed between a liquid pervious sheet such as nonwoven fabric and a liquid impervious sheet such as polyolefin. In recent years, it has been demanded to prepare these sanitary goods as compact type and slim type. Thus, it is required to improve performance and shape stability of the water-retentive material in the absorbent members.
Absorbent materials of powdered polymer and absorbent materials of fibrous polymer have been known conventionally as a water-retentive material, and as described in "Journal of Industrial Materials", Vol.42, No.4, p.18, generally, absorbent materials of powdered polymer are used.
As the absorbent members of powdered polymer, it has been known synthetic polymers such as polyacrylate based compounds and polyvinyl based compounds as well as natural polymers such as cyanomethyl cellulose and carboxymethyl cellulose.
As the absorbent members of fibrous polymer, the following fibers have been known; a fiber produced by a process of mixing sodium salt of carboxymethyl cellulose with viscose prior to spinning, as described in Japanese Patent Laid-open (kokai) No. 56-9418; a fiber produced by a process of carboxymethylating regenerated cellulose fiber, as described in Japanese Patent Publication (kokoku) No.60-2707; and a fiber of a bilayer structure, produced by hydrolyzing an acrylonitrile fiber, thereby forming a polyacrylate based absorbent layer on the outer surface, as described in Japanese Patent Laid-open (kokai) No.55-132754.
For using such water-retentive materials in absorbent members of sanitary goods such as disposable diaper and sanitary napkin, the materials are required to have high absorbency. Furthermore, it is also required that the water-retentive materials have a property such that water once absorbed into the materials should not be released from the materials even under pressure, namely so-called high water retentivity.
For using the fibrous water-retentive materials as the water-retentive materials in absorbent members, the fibrous water-retentive materials are required to have a fiber strength of about 0.8 g/denier (g/d) at their dry state, from the respect of handling of the fibrous water-retentive materials at manufacturing stages.
However, such powdered water-retentive materials come off easily from the absorbent members. The water-retentive materials turn into a gel state with high fluidity in a water-absorbed state, disadvantageously, so such materials are poor in terms of shape stability.
For using the powdered water-retentive materials as a water-retentive material in absorbent members of disposable diaper and the like, the water-retentive materials turn into a gel state within the disposable diaper, when the water-retentive materials absorb urine. Following the motion of a wearer with such disposable diaper thereon, the gel makes a sift with the resultant uneven distribution of the gel in the absorbent member. Additionally, the gel is sticky. Therefore, the wearer feels unpleasant touch and poor feeling during use.
Because the viscose and carboxymethyl cellulose in the fibrous water-retentive material produced by mixing the sodium salt of carboxymethyl cellulose with viscose are both cellulose base, these are highly compatible with each other. Therefore, the water-retentive material has good characteristics as fiber. However, the water retentivity is not sufficient.
In the fibrous water-retentive material produced by carboxymethylating rayon, because the fiber has water absorbency as a whole, the fiber of itself turns into a gel state when the material absorbs water. Accordingly the material are poor in terms of shape stability. Disadvantageously, the fiber strength is low in a dry state.
The fibrous water-retentive material of such bilayer structure, produced by forming a polyacrylate based absorbent layer on the outer surface of an acrylonitrile based fiber, is disadvantageous in that the process of producing the water-retentive material is complex.
In accordance with the present invention, the aforementioned problems are to be solved. The present invention provide a fiber of high water retentivity which is safe for use as absorbent members of sanitary goods such as disposable diaper and sanitary napkin, which also has a high water retentivity, greater shape stability because the fiber can retain the fiber shape even in a water-absorbed state, and a fiber strength sufficient enough for handling at its dried state, as well as an absorbent member wherein the fiber of high water retentivity is used.